彗星知识 QA

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What are comets made of?

Comets are made of a little bit of rock, a few different types of ice (including water, carbon dioxide, ammonia, and methane), and a little bit of complex organic molecules.  They are often described as "dirty snowballs," and their shape and composition always remind me of the old, dirty, icy snow you get on the sides of city roads at the very end of winter.
When are comets visible?

Comets spend most of their time far away from the sun, in the very cold outer reaches of the solar system, only coming in close to the sun every few thousand or even every few million years.

Comets are most easily visible when they get close to the sun, and the many different types of ices on their surfaces start to sublimate (turn from solid into gas).  This makes the comet turn from a small, dark rock into a huge, puffy cloud of gas and dust, making it much easier to see with telescopes or sometimes even with just your eyes.  As the comet gets warmer, it grows a tail of gas and dust that can be millions of miles long, always pointing away from the sun.

How are comets formed?

Comets are little bits of ice left over from the beginning of our solar system.  They formed in the same disk of gas and dust that formed the eight planets, but because they formed in the very cold regions far away from the sun, they include a lot more ice.

Astronomers are still measuring comet composition and running computer simulations to learn exactly where and how comets formed, but they likely formed in the same region of our solar system where Kuiper Belt Objects like Pluto orbit today, and got scattered out onto huge, elliptical orbits when the giant planets changed positions very early in the history of the solar system.

Where do comets come from?

The comets that we occasionally see in the night sky usually come from the really distant reaches of the solar system, where they spend most of our time.  This region is called the Oort Cloud, and it extends from the Kuiper Belt basically halfway to the closest star.  Comets can travel out to hundreds of thousands of times the distance between the Earth and the sun, and then slowly travel back into the inner solar system on million-year-long orbits.

These comets are way too small and faint to be seen until they start to melt close to the sun, and you may be lucky enough to get to see a handful of bright Oort Cloud comets over the course of your lifetime.  The most famous comet, Halley's Comet, is on a much closer orbit and is getting rapidly fainter since it melts every 76 years and has lost a lot of its volatile material.

How can amateur astronomers view comets?

There are always a few faint comets visible somewhere in the night sky if you have access to decent binoculars or a small telescope.  A great database with finder charts is available at in-the-sky.org. The next bright comet could be Comet Tsuchinshan-ATLAS in the fall of 2024, but we won't know how bright it can become until it makes its close approach to the sun and starts sublimating impressively, or not.

Comet orbits

Astronomers classify comets based on the duration of their orbits around the sun. Short-period comets need roughly 200 years or less to complete one orbit, long-period comets take more than 200 years, and single-apparition comets are not bound to the sun, on orbits that take them out of the solar system. Recently, scientists have also discovered comets in the main asteroid belt — these main-belt comets might be a key source of water for the inner terrestrial planets.

Scientists think short-period comets, also known as periodic comets, originate from a disk-shaped band of icy objects known as the Kuiper Belt beyond Neptune's orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets. Long-period comets are thought to come from the nearly spherical Oort Cloud even further out, which get slung inward by the gravitational pull of passing stars. In 2017, scientists found there may be seven times more big long-period comets than previously thought.

Some comets, called sun-grazers, smash right into the sun or get so close that they break up and evaporate. Some researchers are also concerned that comets may pose a threat to Earth as well.

Scientists think short-period comets, also known as periodic comets, originate from a disk-shaped band of icy objects known as the Kuiper Belt beyond Neptune's orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets. Long-period comets are thought to come from the nearly spherical Oort Cloud even further out, which get slung inward by the gravitational pull of passing stars. In 2017, scientists found there may be seven times more big long-period comets than previously thought.

Some comets, called sun-grazers, smash right into the sun or get so close that they break up and evaporate. Some researchers are also concerned that comets may pose a threat to Earth as well.

Comets through history

In antiquity, comets inspired both awe and alarm, "hairy stars" resembling fiery swords that appeared unpredictably in the sky. Often, comets seemed to be omens of doom — the most ancient known mythology, the Babylonian "Epic of Gilgamesh," described fire, brimstone, and flood with the arrival of a comet, and the Roman emperor Nero saved himself from the "curse of the comet" by having all possible successors to his throne executed. This fear was not just limited to the distant past — in 1910, people in Chicago sealed their windows to protect themselves from what they thought was the comet's poisonous tail.

For centuries, scientists thought comets traveled in the Earth's atmosphere, but in 1577, observations made by Danish astronomer Tycho Brahe revealed they actually traveled far beyond the moon. Isaac Newton later discovered that comets move in elliptical, oval-shaped orbits around the sun, and correctly predicted that they could return again and again.

Chinese astronomers kept extensive records on comets for centuries, including observations of Halley's Comet going back to at least 240 B.C., and historic annals that have proven valuable resources for later astronomers.

Missions to comets

Several missions have ventured to comets.

NASA's Deep Impact collided an impactor into Comet Tempel 1 in 2005 and recorded the dramatic explosion that revealed the interior composition and structure of the nucleus. The mission also included a flyby of Comet Hartley 2 and the remote sensing of Comet Garradd during an extended mission.

In 2009, NASA announced that samples returned from Comet Wild 2 during the Stardust mission revealed a building block of life — glycine.  It was the first time an amino acid was found in a comet.

In 2014, the European Space Agency's Rosetta spacecraft entered orbit around Comet 67P/Churyumov-Gerasimenko. The Philae lander touched down on Nov 12, 2014. Among the Rosetta mission's many discoveries was the first detection of organic molecules on the surface of a comet; a strange song from Comet 67P/Churyumov-Gerasimenko; the possibility that the comet's odd shape may be due to it spinning apart, or resulting from two comets fusing together; and the fact that comets may possess hard, crispy outsides and cold but soft insides, just like fried ice cream. On Sept. 30, 2016, Rosetta intentionally crash-landed on the comet, ending its mission.

June 19, 2019, ESA selected the Comet Interceptor mission as the last "fast" or F-class mission. The new mission will intercept an as-yet-undiscovered comet as it enters the inner solar system. The mission consists of three spacecraft that will capture snapshots of the comet from different angles, creating a 3D profile of the object and characterizing its surface, composition, shape and structure. Comet interceptor is due to launch in 2029.

Famous comets

Halley's Comet is likely the most famous comet in the world, even depicted in the Bayeux Tapestry that chronicled the Battle of Hastings of 1066. It becomes visible to the naked eye about every 75 years when it nears the sun. When Halley's Comet zoomed near Earth in 1986, five spacecraft flew past it and gathered unprecedented details, coming close enough to study its nucleus, which is normally concealed by the comet's coma.

The roughly potato-shaped, 9-mile-long (15 km) comet contains equal parts ice and dust, with some 80% of the ice made of water and about 15% of it consisting of frozen carbon monoxide. Researchers believe other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet was unexpectedly extremely dark black — its surface, and perhaps those of most others, is apparently covered with a black crust of dust over most of the ice, and it only releases gas when holes in this crust expose ice to the sun.

The comet Shoemaker-Levy 9 collided spectacularly with Jupiter in 1994, with the giant planet's gravitational pull ripping the comet apart for at least 21 visible impacts. The largest collision created a fireball that rose about 1,800 miles (3,000 km) above the Jovian cloud tops as well as a giant dark spot more than 7,460 miles (12,000 km) across— about the size of the Earth — and was estimated to have exploded with the force of 6,000 gigatons of TNT.

A highly visible comet was Hale-Bopp, which came within 122 million miles (197 million km) of Earth in 1997. Its unusually large nucleus gave off a great deal of dust and gas — estimated at roughly 18 to 25 miles (30 to 40 km) across — appeared bright to the naked eye.

Comet ISON was expected to give a spectacular show in 2013. However, the sun-grazer did not survive its close encounter with the sun and was destroyed in December of the same year.

In 2021, scientists found what could be the largest comet ever seen. Comet C/2014 UN271 or Bernardinelli-Bernstein after its discoverers, University of Pennsylvania graduate student Pedro Bernardinelli and astronomer Gary Bernstein, was officially designated a comet on June 23. Astronomers estimate this icy body has a diameter of 62 miles to 124 miles (100 km to 200 km), making it about 10 times wider than a typical comet. The comet will make its closest approach to our planet in 2031 but will remain at quite a distance even then.